The present invention relates to a light beam deflecting device equipped with a polygon mirror (rotating polygon mirror) used for an image forming apparatus such as, for example, a laser beam printer and a laser facsimile machine.
Heretofore, in an image recording apparatus such as a laser printer, a laser beam is made to enter a polygon mirror (rotating polygon mirror) that rotates at high speed on a light beam deflecting device based on information read as a means of writing images, and thus, a reflected light beam is made to scan to be projected on the surface of a photoreceptor, for image recording. FIG. 7 is a perspective view showing an embodiment of a beam-scanning optical device employing a light beam deflecting device using a polygon mirror.
In the drawing, the numeral 80 represents a semiconductor laser, 81 represents a collimator lens of a beam-forming optical system, 82 represents a first cylindrical lens, 83 represents a polygon mirror, each of 84A and 84B is an fθ lens, 85 represents a second cylindrical lens, 86 represents a mirror, 87 represents a cover glass and 88 represents a photoreceptor drum. Incidentally, 89 represents an index mirror for synchronous detection, 89S represents an index sensor for synchronous detection and 83M represents a driving portion to rotate polygon mirror 83 on the light beam deflecting device.
A light beam emitted from the semiconductor laser 80 is made to be collimated light by the collimator lens 81, and passes through the first cylindrical lens 82 of a first image forming optical system to enter a mirror surface of the polygon mirror 83 that is rotating at constant high speed. Then, the reflected light therefrom passes through a second image forming optical system composed of fθ lenses 84A, 84B and the second cylindrical lens 85 to conduct (main) scanning on the circumferential surface of the photoreceptor drum 88 with a spot having a prescribed diameter, through the mirror 86 and the cover glass 87. The main scanning direction is subjected to fine adjustment by an unillustrated adjusting mechanism, and synchronous detection for each one line is made when a beam before the start of scanning enters index sensor 89S through index mirror 89.
To obtain an excellent latent-image on photoreceptor drum 88 with the beam-scanning optical device like that mentioned above, it is requested that a reflecting surface of the polygon mirror that rotates at high speed is processed to be a highly accurate plane, and it rotates without eccentricity and inclination from a rotation axis and without positional deviation in the axial direction.
A rotary mirror unit is provided with a polygon mirror and a permanent magnet for generating torque that faces a coil mounted on a printed board on the fixed base board, and it is structured to rotate at high speed on a bearing.
When the mirror unit rotates especially at high speed such as 30,000 rpm or higher, an air dynamic pressure bearing is used, and there is known a light beam deflecting device wherein a ceramic material is used for a member on one side of a dynamic pressure bearing member-and a metallic material is used for a member on the other side, to raise durability and abrasion resistance and thereby to lengthen life, between an inner cylindrical bearing and an outer cylindrical bearing both forming the air dynamic pressure bearing (JP TOKKAI No. 2001-173643).
There is further known a light beam deflecting device wherein a surface roughness of a contact surface on one side or both sides where a polygon mirror and a flange are in contact each other satisfies 3 μm≦Ry≦20 μm, under the structure where the polygon mirror is pressed against the flange to be held by a pressing member (JP TOKKAI No. 2002-48997).
However, in the light beam deflecting device which has been used so far, if the polygon mirror is shifted by a centrifugal force or by thermal expansion while it is used, a balance of a rotor portion is changed to cause vibrations and noises. If an amount of shifting grows greater, binding is caused on the dynamic pressure bearing, resulting in its rupture. Further, if spring force (force of an elastic member) which presses the polygon mirror is made to be stronger, the polygon mirror becomes hard to be shifted, but a flatness of the polygon mirror is worsened, and scanning characteristics, namely, image qualities are deteriorated, which is a problem. If contact surfaces between the polygon mirror and the flange portion are roughened, the polygon mirror turns out to be hardly shifted, but it becomes insufficient in use at high speed rotation of 50,000 rpm or higher or at high temperature of 60° C. or higher. Therefore, when the polygon mirror is made to adhere for prevention of its shifting, if hard adhesives of an epoxy type are used, there is caused a problem that the polygon mirror is distorted by internal stress of adhesives and by thermal strain in the course of adhesion hardening or in the course of usage, although it is effective for preventing shifting of the polygon mirror. When flexible adhesives of a silicon type are used, it is impossible to control shifting of the polygon mirror sufficiently, although distortion of the polygon mirror can be prevented by deformation of adhesive itself.